Histopathology of Avocado Fruit Infected by Avocado Sunblotch Viroid

Abstract

Avocado sunblotch viroid (ASBVd) was detected in the exocarp and mesocarp of symptomatic and asymptomatic 'Hass' avocado fruit. Macroscopic symptoms caused by the ASBVd in fruits are the result of anatomical and chemical changes in the structure of the exocarp and mesocarp cells, which are typified by marked cellular disorganization, accumulation of phenolic compounds in the cytoplasm and cell walls, and reduction of cytoplasmic content resulting in cell collapse and death. The mesocarp of symptomatic fruits showed a reduction of 28-38% in chlorophyll A and 23-28% in chlorophyll B content, as well as an increase of up to 50-62% in phenolic compounds compared to asymptomatic fruit. The phloem cells of symptomatic fruit showed hyperplasia and the xylem elements were occluded in the mesocarp. Additionally disorganization and cell collapse was observed leading to groove formation in symptomatic fruit. Chlorophyll reduction and increase in phenolic compounds probably leads to the development of yellow or red symptoms in the rind. Asymptomatic fruits did not show any anatomical change despite the presence of the ASBVd. The PCR and dot-blot hybridization membranes showed relatively higher signal intensity in symptomatic fruits in contrast to the asymptomatic ones.